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>The Anatomy Of A Widget</A
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><P
>In order to create a new widget, it is important to have an
understanding of how GTK objects work. This section is just meant as a
brief overview. See the reference documentation for the details. </P
><P
>GTK widgets are implemented in an object oriented fashion. However,
they are implemented in standard C. This greatly improves portability
and stability over using current generation C++ compilers; however,
it does mean that the widget writer has to pay attention to some of
the implementation details. The information common to all instances of
one class of widgets (e.g., to all Button widgets) is stored in the 
<I
CLASS="EMPHASIS"
>class structure</I
>. There is only one copy of this in
which is stored information about the class's signals
(which act like virtual functions in C). To support inheritance, the
first field in the class structure must be a copy of the parent's
class structure. The declaration of the class structure of GtkButtton
looks like:</P
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>struct _GtkButtonClass
{
  GtkContainerClass parent_class;

  void (* pressed)  (GtkButton *button);
  void (* released) (GtkButton *button);
  void (* clicked)  (GtkButton *button);
  void (* enter)    (GtkButton *button);
  void (* leave)    (GtkButton *button);
};</PRE
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>When a button is treated as a container (for instance, when it is
resized), its class structure can be cast to GtkContainerClass, and
the relevant fields used to handle the signals.</P
><P
>There is also a structure for each widget that is created on a
per-instance basis. This structure has fields to store information that
is different for each instance of the widget. We'll call this
structure the <I
CLASS="EMPHASIS"
>object structure</I
>. For the Button class, it looks
like:</P
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CLASS="PROGRAMLISTING"
>struct _GtkButton
{
  GtkContainer container;

  GtkWidget *child;

  guint in_button : 1;
  guint button_down : 1;
};</PRE
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>Note that, similar to the class structure, the first field is the
object structure of the parent class, so that this structure can be
cast to the parent class' object structure as needed.</P
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